The problem i see with this is that it would be at least mandatory to have at least every "cross class trait slot" filled.
And this perhaps/most likely will return us just to the cross class skills of the past, when every physical DPS had to level DRG just for "Invigorate" and "Blood for Blood" or every caster had to level BLM for "Swiftcast" for example.

To take the example from OP:
Quote Originally Posted by Malkria View Post
One simple idea I've had is having 'traits' you unlock at level 50, 60, and 70 that you can equip for small bonuses.
This would be "bad", sorry i don't have a better word for this at the moment.
Level 50, 60 and 70 would be to much to level, even if most players will say "it's easy and fast to level".
A "better" and more newcomer freindly aproach would be that the classes/jobs only give like 1 to 2 traits.
The first with level 15 or unlocking the job for the advanced jobs.
The second for unlocking the job or for completing the first level cap they can hit (50 for DRK/MCH/AST and 60 for SAM/RDM).
Further down the road you can place variations or buffed versions of the traits.

I know my idea is not "better" then others just builed apon them and maybe they just offer a different point of view or a new aproach.